Solar District Heating

Solar district heating (SDH) plants are a very large scale application of the solar thermal technology.

These plants are integrated into local district heating networks for both residential and industrial use. During warmer periods they can totally replace other sources, usually fossil fuels, used for heat supply. Thanks to the developments in large scale thermal storage it is now also possible to store heat in summer for winter use. Solar heat can also meet a share of the heating demand during the winter.

The economic and environmental benefits derived from the acknowledged reliability of this solar heat application, allied to the technical expertise gained over decades, have contributed to the growing interest in its commercial operation. Currently there are many plants in operation in Sweden, Denmark, Germany and Austria.

District heating is a network providing heat, usually in form of hot water. This heat is mainly used for space heating and for domestic hot water (drinkable water); however, It usually also meets some industrial needs. District heating systems can serve whole cities; when a system is limited to a group of buildings it is referred to as block heating. The main advantage of these systems is that the large district heating plants are more efficient, more economic and create less pollution than decentralized fossil fuel based boilers. The heat generated in a centralized manner is then distributed to urban areas through a system of pipelines specially designed for transporting heat, which is then supplied house to house.

In a district heating system, the heat is generated on a larger scale. Therefore, solar thermal, as other technologies, can be scaled up to provide large quantities of hot water. Hence, solar district heating (SDH) plants are a very large scale application of conventional solar thermal technology. These plants are integrated into local district heating networks for both residential and industrial use. During warmer periods they can wholly replace other sources, usually fossil fuels, used for heat supply. Thanks to developments in large scale thermal storage, it is now also possible to store heat in summer for winter use. Solar thermal can also meet a share of the heating demand during the winter.

Operation & Maintenance: The operating and maintenance requirements are in line with operating such large systems, either solar thermal or using other technologies. Correct operation is also required to maximize performance.

These systems consist of solar thermal plants, made up of hundreds of solar thermal collectors. Considering the requirements of such large systems, larger collectors working with bigger loads have been designed specifically for such application. For smaller systems (block heating), normal solar thermal collectors, either flat plate, evacuated tube or even concentrating, can be used. These solar thermal plants supply heat to a district heating network. It can consist of a centralized supply, where a very large collector field delivers heat to a main heating central. It can also provide, directly or indirectly, a large seasonal heat store that will contribute to increasing the input of solar thermal plant to the whole system.

The other possible configuration is a decentralized supply or distributed solar district heating. In this case, solar collectors are placed at suitable locations (buildings, parking lots, small fields) and connected directly to the district heating primary circuit on site. This solution can also be interesting for smaller district heating networks or block heating networks. A system is considered as very large when it is over 350 kWth (500 m²) but solar district heating systems can reach sizes 300 times bigger, i.e. over 100 MWth.

The benefits of solar thermal systems, in particular for such large systems, cover environmental, political and economic aspects. Environmental benefits relate to the capacity of reducing harmful emissions. The reduction of CO2 emissions depends on the quantity of fossil fuels replaced directly or indirectly, when the system replaces the use of carbon-based electricity used for water heating. Depending on the location, a system of 1.4 MWth (2000 m²) could generate the equivalent of 1.1 GWhth /year, a saving of around 175 kg of CO2. Political and economic benefits are associated with the potential savings in energy costs and the possibility of improving energy security by reducing energy imports, while creating local jobs related to the manufacturing, commercialization, installation and maintenance of solar thermal systems. Regarding energy costs, and potential savings, there are three main aspects to consider that have a bigger impact on the comparable costs of the energy produced by a solar thermal system. These are the initial cost of the system, the lifetime of the system and the system performance. These factors depend on the location (affecting climate, insulation, taxes, cost of living, etc.) and quality of the system (affecting performance, lifetime and cost). This can vary significantly from country to country. Therefore, average investment costs for solar thermal systems can vary greatly from country to country and between different systems. According to the IEA, for large systems in Europe, the investment costs can go from 315 to 930 EUR/kWth. In terms of energy costs, it can range from 20 to 70 USD/MWhth (18 to 63 EUR/MWhth) in Southern United States and between 35 and135 EUR/MWhth in Europe.

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